This occurs when you have a missing or incorrect API_KEY, which is read out of your GoogleService-Info.plist file. I would check the following:
1) Confirm your GoogleService-Info.plist file contains an API_KEY. There is a rare bug where you can download the file and this field is not inserted. If the API_KEY is missing, download a new GoogleService-Info.plist file (click on your project, click the gear icon in the upper left of the screen, click project settings, select your app, and download a new config file).
2) Confirm you're using the correct GoogleService-Info.plist file. If you setup two projects, make sure both projects are using different files, and they aren't mixed up.
3) Make sure your GoogleService-Info.plist file wasn't renamed to something like GoogleService-Info.plist (1) by your web browser. If it was, rename it back to GoogleService-Info.plist
Edit: This can also be due to a CFBundleVersion or CFBundleShortVersionString not being a major.minor.patch, all numeric. The restriction on CFBundleVersion is getting dropped soon (along with a better error message)